Microminiature capacitors whose capacitance values can be controllably varied are used in a variety of applications of practical importance. Thus, for example, voltage-controlled capacitors (varactors) are employed as tunable components in circuits such as voltage-controlled oscillators. They are also used as discrete tunable elements in filters and loaded-line phase shifters.
Micromachining techniques have been utilized to fabricate very small variable capacitors for high-frequency use. (See, for example, "A Micromachined Variable Capacitor For Monolithic Low-Noise VCOS" by Young and Boser, Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 2-6, 1996, pages 86-89.) But heretofore it has proven difficult, if not impossible, to micromachine a variable capacitor that exhibits a sufficiently high quality factor (Q) and a wide enough tuning range to satisfy the requirements of some important high-frequency applications. In some such applications, operation in the gigaHertz range with a monotonic capacitance-versus-voltage characteristic and a high Q (1000 or higher) may be specified. Furthermore, in some such cases, a wide tuning range (for example, 0.1-to-5 picoFarads) may be required.
Accordingly, continuing efforts have been directed by workers skilled in the art aimed at trying to provide an improved microminiature variable capacitor. In particular, these efforts have focussed on attempting to provide a reliable, low-cost micromachined variable capacitor exhibiting advantageous characteristics such as linearity, high Q and a wide tuning range. It was recognized that these efforts, if successful, could provide an improved variable capacitor for use in a variety of practical high-frequency applications.